The present invention relates to a method of producing a spheroidal graphite cast iron article suffering from little volume shrinkage.
Since spheroidal graphite cast iron has excellent mechanical strength, it is widely used in various applications including automobile parts, machine parts, etc. Also, because of recent trend of increasing fuel efficiency of automobiles, it is proposed to make cast iron products thinner. However, since the cast iron products as automobile parts are composed of thin portions and thick portions in many cases, it is highly likely that when cast by a conventional method, shrinkage cavities are generated in the cast products. To prevent the generation of shrinkage cavities, a large riser may be used. However, a large riser leads to a higher production cost of spheroidal graphite cast iron articles. In addition, there is a problem that chill is likely to appear in a thin portion of the spheroidal graphite cast iron article, resulting in poor mechanical strength.
To prevent the generation of shrinkage cavities in a thick portion, it is necessary to suppress the volume shrinkage of the spheroidal graphite cast iron article in the process of solidification from the melt. The shrinkage cavities can be suppressed by inoculating a material which forms nuclei for the precipitation of graphite. In this case, graphitization is accelerated by the inoculation of the above material, and the resulting graphite serves to expand the volume of the spheroidal graphite cast iron article. This is the mechanism of preventing the generation of shrinkage cavities.
Also, when the precipitation of graphite is accelerated in a thin portion, the generation of chill is prevented, resulting in higher mechanical strength.
Conventionally used as a material of forming nuclei for the precipitation of graphite is a combination of a lanthanide element and sulfur. However, sulfur in a molten state in the melt of a spheroidal graphite cast iron fails to generate a sufficient amount of nuclei. Accordingly, the addition of a lanthanide element and sulfur cannot provide a spheroidal graphite cast iron article with improved mechanical strength.